Wind energy installations with pitch-adjustable rotor blades generally have, for reasons of operational reliability, an emergency drive device with an autarkic energy store, such as a rechargeable battery. This emergency drive device with its own energy store is used as a reserve for incidents, to be precise in particular if the main system for the pitch adjustment drive no longer operates on account of a defect or on account of a failure of the energy supply. The emergency drive device is provided in order to be able to reliably shut down the wind energy installation even under those conditions. DE-B-103 35 575 discloses a pitch adjustment drive having an emergency drive device. The pitch adjustment drive comprises a mains-operated converter driving a three-phase motor which adjusts the angle of incidence on its associated rotor blade or blades. Furthermore, a rechargeable battery as an independent energy source is provided as the emergency drive device. The rechargeable battery is connected to the converter such that, in the case of a defect, the rechargeable battery is connected to the intermediate circuit of the converter via a switch and thus makes available the energy required to actuate the motor. It has been shown, however, that when the emergency drive device is in operation, certain load states can occur in which the rotor blade to be adjusted drives the motor. The result is a shift state. The latter leads to electric energy being generated in the motor, which can result in feedback of power to the rechargeable battery. Since in this case large intensities of current can occur, there is a risk of high-current charging for the rechargeable battery. Additionally this produces short-term charging cycles which decrease the service life of the rechargeable battery. It increases wear of the rechargeable battery, and there is even a danger of premature failure.